1. Field of the Invention
The present invention relates to an abnormality detection apparatus for an exhaust gas purification apparatus, which is used to detect an abnormality in an exhaust gas purification apparatus having an NOxstorage reduction catalyst and an NOxselective reduction catalyst.
2. Description of the Related Art
A conventional exhaust gas purification apparatus provided in an exhaust passage of a lean burn internal combustion engine capable of operating at a lean air-fuel ratio that is higher than the stoichiometric air-fuel ratio may include an NOxstorage reduction catalyst (also referred to hereafter as an NSR catalyst) and an NOxselective reduction catalyst (also referred to hereafter as an SCR catalyst). In this type of exhaust gas purification apparatus, the SCR catalyst is disposed on a downstream side of the NSR catalyst in the direction of a flow of exhaust gas flowing through the exhaust passage.
When a peripheral atmosphere of the NSR catalyst is an excessive oxygen atmosphere, the NSR catalyst occludes NOx contained in the exhaust gas. When an air-fuel ratio of the exhaust gas flowing into the NSR catalyst decreases to an extent at which the peripheral atmosphere becomes a reducing atmosphere, the NOx occluded to the NSR catalyst is reduced. Further, when the peripheral atmosphere of the NSR catalyst becomes a reducing atmosphere, HC and H2 in the exhaust gas react with NOx such that NH3 is generated in the NSR catalyst. In the exhaust gas purification apparatus described above, the NH3 is adsorbed to the SCR catalyst. The SCR catalyst selectively reduces the NOx contained in the exhaust gas using the adsorbed NH3 as a reducing agent.
Japanese Translation of PCT International Application Publication No. 2009-517599 discloses an exhaust gas purification apparatus having an NSR catalyst and an SCR catalyst disposed on the downstream side of the NSR catalyst. In this exhaust gas purification apparatus, a NOx sensor is provided on an upstream side of the NSR catalyst and a downstream side of the SCR catalyst.